AISI 4140 Steel
In the hierarchy of high-strength engineering materials, AISI 4140 steel (UNS G41400) is a legendary “all-rounder.” Often referred to as Chromoly steel, this chromium-molybdenum alloy is favored for its exceptional toughness, high fatigue strength, and ability to be heat-treated to a wide range of hardness levels.
Whether you are designing heavy-duty gears, high-tensile bolts, or oil and gas components, understanding the metallurgical profile of 4140 is essential for ensuring project longevity and safety. AISI 4140 steel is a low alloy steel that contains chromium, molybdenum and manganese. It is one of the most commonly used medium carbon steels, and has high strength, fatigue resistance and hardness properties.
AISI 4140 steel is often used in applications such as shafts, gears, bolts and other automotive components due to its excellent mechanical properties. AISI 4140 steel has an impressive combination of strength, ductility and shock resistance making it suitable for use in demanding industrial environments.
AISI 4140 steel Specification
The UNS Number of AISI 4140 Steel is UNS G41400. AISI 4140 steel material specifications are:
- ASTM A 193: Grade B7, B7M,
- ASTM A29, A 322, A331, A506, A513, A519, S646, A 752: Grade 4140,
- SAE J404: Grade 4140,
- AMS 6349, 6381,
- EN 10250: Grade 42CrMo4 (1.7225),
- BS 970: Grade 42CrMo 4,
- DIN 17200: Grade 42CrMo4 (1.7225),
AISI 4140 steel Equivalent material
Belonging to the family of SAE steels—the American iron and steel institute (AISI) classifies it as a 41xx series—several equivalent materials can be found on the market today. AISI 4140 equivalent materials are:
- Grade 42CrMo4 (1.7225),
- JIS G4105- Grade SCM440,
- AS 1444-Grade 4140
- ASTM A29, A 322, A331, A506, A513, A519, S646, A 752: Grade 4140
4140 steel meaning
| Number | Meaning |
| 4 | means that aisi 4140 steel is Molybdenum as main alloying |
| 1 | Means added Chromium for alloying. |
| 40 | Grade designation. |
AISI 4140 Chemical Compositions
4140 steel contains several key chemical components that give it its unique properties such as molybdenum, silicon, manganese, chromium and carbon. These elements provide AISI 4140 with increased wear resistance, fatigue strength and ductility compared to other grades of steel.
The chemical composition of AISI 4140 consists mainly of iron along with small amounts of molybdenum (0.15-0.25%), silicon (0.15-0.30%), manganese (0.75-1%), chromium (0.8-1.1%) and carbon (0.38- 0.42%).
The “41” in the AISI designation signifies that it belongs to the Chromium-Molybdenum family. The “40” indicates a nominal carbon content of 0.40%. This specific balance of elements allows for deep through-hardening that simpler carbon steels cannot achieve.
Detailed chemical composition of AISI 4140 steel is given below:
| Grade | C% | Si | Mn% | Si% | S% | P% | Cr% | Mo% |
| 4140 | 0.38 to 0.43 | 0.15 to 0.35 | 0.75 to 1.0 | 0.15 to 0.35 | 0.040 | 0.035 | 0.8 to 1.10 | 0.15 to 0.25 |
AISI 4140 Mechanical Properties
AISI 4140 is most commonly used in the Annealed or Quenched and Tempered (Q&T) condition. Its mechanical performance is highly dependent on the tempering temperature.
4140 steel is a versatile quenched and tempered steel (here annealed version) with high strength and high toughness, which is often used for highly stressed components in vehicle construction.
It is also universally used in mechanical engineering in the quenched and additionally surface-hardened condition. AISI 4140 steel tensile strength is 95 Ksi (655 MPa) and Yield strength is 60.2 Ksi (415 MPa) minimum.
The hardness of 4140 steel varies based on the toughness temperature and usually in the range of 217 HB to 241 HB.
| Properties | Value |
| Tensile Strength | 95 Ksi (655 MPa) |
| Yield Strength | 60.2 Ksi (415 MPa) |
| % Elongation | 25.70 |
| Density | 7.86 g/cm3 |
| Melting Points | 2580 to 2650 °F |
Typical Properties (Oil Quenched & Tempered @ 600 Deg C
- Tensile Strength: 850-1000 MPa (123,000-145,000 psi)
- Yield Strength: > 650 MPa (95,000psi)
- Elongation: 12-18%
- Hardness: 28-34 HRC (Brinell approx 275-320 HB)
- Modulus of Elasticity: 190-210 GPa
AISI 4140 Heat Treatment
In the quenched and tempered condition, AISI 4140 steel has good mechanical properties and can be used for a wide variety of construction parts in automotive and vehicle construction.
The tempering process consists of hardening and subsequent tempering of the material.
In this step, the material processed in this way receives a previously precisely defined high tensile and fatigue strength, because the tempering heat treatment can be used to influence the relationship between hardness and toughness of the steel in a targeted manner.
This steel is hardened by keeping the temperature in a range between 820°C and 860°C.
The quenching – i.e., the rapid cooling of the workpiece – then takes place in an oil bath or in water. The tempering temperature following hardening depends on the desired strength.
Hardened steel becomes softer the higher temperature you use for tempering. It should be noted that tempering must be carried out as soon as possible after the hardening process in order to avoid the formation of cracks.
A tempering treatment in a temperature range between 540°C – 680°C with subsequent cooling in air is recommended.
- Annealing: at 800°C – 850°C, hold & furnace cool.
- Normalizing: at 870°C – 900°C, holding 10-15 minutes followed by still air cooling.
- Quenching: at 820°C – 860°C, hold and followed by water quenching.
- Tempering: at 540°C – 980°C. soaking 1 hour per inch thickness followed by still air cooling.
AISI 4140 Steel Welding
The most important factor when welding AISI 4140 steel is preheating the material before joining two pieces together.
4140 steel material can be welded with Stick welding, TIG and MIG Welding.
Click here for AISI 4140 Steel Welding Procedure.
From a welding engineering perspective, AISI 4140 is classified as having limited weldability. Its high carbon and alloy content make it extremely susceptible to Cold Cracking or Hydrogen-Induced Cracking (HIC) in the Heat-Affected Zone (HAZ).
- Preheat is Mandatory: Always maintain a preheat between 250°C and $450°C depending on thickness.
- Filler Metal: For high-strength joints, use ER80S-D2 or ER90S-D2. If strength is less critical than ductility, ER70S-2 may be used.
- Post-Weld Heat Treatment (PWHT): Slow cooling is required. Stress relieving at 600°C-650°C immediately after welding is highly recommended to prevent brittle failures.
AISI 4140 Steel Uses
Mechanical engineering in general, machine components, axles, steering knuckles, connecting rods, crankshafts, transmission shafts, pinions, gears, drums, base plates, body parts.
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🛠️ Deep-Dive FAQ: AISI 4140 Steel Technical Queries
What is the difference between AISI 4140 and 4142 steel?
While nearly identical, the primary difference lies in the Carbon content. AISI 4142 has a slightly higher carbon range (0.40-0.45) compared to 4140 ($0.38-0.43%). This allows 4142 to achieve slightly higher through-hardenability in thicker sections. In many industrial applications, these two grades are considered interchangeable and are often dual-certified.
Is 4140 steel considered a “Low Alloy” or “High Alloy” steel?
AISI 4140 is classified as a Low Alloy Steel. Even though it contains high-performance elements like Chromium and Molybdenum, the total alloy content is generally below 5%. This category is ideal for applications requiring a balance of high strength, toughness, and relatively good machinability compared to high-alloy tool steels.
What is the Rockwell C hardness of 4140 in the annealed vs. hardened state?
- Annealed Condition: Typically measures around 15-}20 HRC (92 HRB). This is the state used for easy machining and forming.
- Fully Hardened (As Quenched): Can reach up to 54-}59 HRC. However, it is extremely brittle in this state and must be tempered.
- Standard Tempered Condition: Most industrial 4140 is supplied or used in the 28-}32 HRC range for an optimal balance of strength and impact resistance.
Can AISI 4140 be Case Hardened or Induction Hardened?
Yes. Because 4140 has sufficient carbon (0.40%), it responds exceptionally well to Induction Hardening or Flame Hardening. This process allows the core to remain tough and ductile while the outer “skin” reaches a hardness of 50-55 HRC, making it perfect for wear-resistant shafts, gears, and pins.
Why is AISI 4140 often called “709” or “4140HT”?
- 709: This is an older trade designation (often associated with Assab/Uddeholm) commonly used in the Australian and Asian markets to refer to 4140.
- 4140HT: This stands for “Hardened and Tempered.” It signifies that the steel has already undergone heat treatment to a specific tensile range (usually “Condition T”) and is ready for use without further thermal processing.
What is the Machinability Rating of 4140?
In its annealed condition, AISI 4140 has a machinability rating of approximately $65\%$ compared to the 1212 carbon steel standard. When machining in the “Hardened and Tempered” state, cutting speeds must be reduced, and carbide tooling is highly recommended to prevent premature tool wear.
Is AISI 4140 corrosion resistant?
No. While the Chromium content (1%) provides slightly better atmospheric corrosion resistance than plain carbon steel, 4140 is not stainless steel. It will rust if exposed to moisture or corrosive chemicals. For offshore or marine use, 4140 components are typically plated (Zinc, Chrome) or coated with black oxide or phosphate.
What is “Condition T” in 4140 steel?
In British and Australian standards (like AS 1444), “Condition” letters refer to specific tensile strength ranges. Condition T for 4140 typically specifies a tensile strength of 850-}1000 MPa. This is the most common pre-hardened state found in local steel service centers.
How do I avoid “Quench Cracking” when heat treating 4140?
Quench cracking occurs if the cooling is too violent. To prevent this:
- Use Oil: Never water-quench 4140; always use a circulated oil quench.
- Avoid Sharp Corners: Ensure the part has generous radii in the design to reduce stress concentration.
- Immediate Tempering: Transfer the parts to the tempering furnace while they are still “hand-warm” (60-80°C) to relieve internal stresses immediately.
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